This invention relates to the field of switch mode power converters and, in particular, to the field of synchronous rectification for high efficiency converters.
With the ever-increasing demand in the power electronics market for low voltage, high current power converters, power supply designers are faced with the challenge of designing high efficiency converters in smaller physical sizes. For converters with output voltages as low as 2.2V or lower, the state of the art schottky diodes have limited utility because their forward voltage drop of 0.3V is still unacceptably high. To achieve an overall efficiency as high as 90%, the rectification stage voltage drop would have to be lower than 0.1V. Only with synchronous rectification MOSFETs can one possibly approach achieving this goal. The performance of synchronous rectification, however, is not always superior to traditional schottky diode rectification. This is especially true when the driving signal timing and the driving voltage level of synchronous rectifiers are not well designed.
There are two primary methods for controlling synchronous rectifiers: the self-driven method and the controller driven method. In isolated power conversion, the controller driven method is usually more complex and costly than its self-driven counterpart, so it is not preferred.
There are two types of self-driven methods: the voltage driven method and the current driven method. A current driven synchronous rectifier uses current sensing to control the switching times. The current driven rectifier requires additional current sensing components such as current transformer or current sensing MOSFET thereby increasing circuit complexity. A voltage driven synchronous rectification is attractive for its simplicity. The driving signals for the voltage driven synchronous rectifier can be derived from the main transformer windings or output inductor coupled windings.
Among various isolated topologies, the forward topology is one of the most suitable topologies for low voltage power conversion because it is the simplest derivation of isolated step-down topology, however it has shortcomings. A prior art synchronous rectifier, as shown in FIG. 1A, uses the secondary winding of the main transformer to drive the synchronous rectifier. The gates of the synchronous MOSFETs S1 and S2 are connected to two terminals of the main transformer secondary winding. Alternating voltage at the secondary winding drives the MOSFETs S1 and S2 in synchronism with the converter main switch S.
The main drawback of this topology is conduction through the body diode of the synchronous MOSFETs when magnetizing current resets to zero. When this happens, the voltage at the transformer secondary becomes zero as shown in FIG. 1B. The time period when this occurs is normally called the dead time. During this dead time period, a freewheeling synchronous rectifier is not driven on but there is output current flowing through it nonetheless. This is because current is flowing through the body diode of the synchronous rectifier. The body diode of a MOSFET has a higher forward voltage drop and poorer reverse recovery characteristic than a normal fast recovery diode. So during this dead time period, the loss is much higher with the synchronous rectifier than with a traditional diode rectifier. The advantages of synchronous rectifiers is greatly compromised because of diode body conduction during dead time periods.
Efforts to address the body diode conduction problem include the active clamp method as shown if FIG. 2A. With the help of an auxiliary switch SA, which is coupled to a precharged capacitor, magnetizing energy is recovered and transformer dead time can be reduced to a very short time period. The active clamp method is effective and provides complementary signals to drive two synchronous MOSFETs. But, it requires an additional floating switch SA on the primary side, which is costly together with its associated circuitry.
In U.S. Pat. No. 5,886,881, Xia put the active clamp switch on the secondary side. But, to do this a p-channel MOSFET was needed, which has much inferior parameter values than an n-channel MOSFET.
In U.S. Pat. No. 5,343,383, Shinada attempted to increase the response speed of synchronous rectifiers by driving the gates through capacitors. This reference, however, does not attempt to solve the dead time problem and does not prevent MOSFET body diode conduction when in the presence of transformer leakage inductance.
Other systems have proposed methods for keeping synchronous rectifiers turned on during dead time. But, they attempt this by reducing the input voltage range of the converter.
Therefore, there remains a need in this art for a system for overcoming the body diode conduction problem in synchronous rectifiers without adding costly components, inferior components and without reducing the input voltage range of the converter.
The present invention overcomes the problems noted above and satisfies the needs in this field for a system for overcoming the body diode conduction problem in synchronous rectifiers during dead time.
The present invention provides a simple system for eliminating the problem of body diode conduction without using the active clamp method. The present invention utilizes a retention of gate charge technique wherein the gate charge is retained during dead time until it is released at the end of a switching period. As a result, the synchronous rectifier remains turned on during dead time without a primary clamp circuit and without body diode conduction.
Because transformer leakage inductance may delay the switching of the synchronous MOSFETs, the present invention provides a system that employs an auxiliary winding to drive the synchronous MOSFETs.
The present invention is very versatile since it can be used in many topologies. Disclosed are several embodiments for use with current doubler topologies, topologies with center-tapped secondary windings, and forward converter topologies.
Accordingly, it is an object of the present invention to provide high efficiency self-driven synchronous rectifier circuits for low voltage power supply apparatus.
It is another object of the present invention to use the voltage sensing method for its simplicity.
It is another object of the present invention to enable synchronous rectifiers to remain conducting during dead time without using active clamp circuit on the primary side.
It is another object of the present invention to avoid the effect of leakage inductance in the main transformer of a converter.
It is another object of the present invention to provide synchronous rectification to a wide range of power converter topologies.
In accordance with the present invention a synchronous rectifier system for a power converter is provided. The system comprises a transformer having a first secondary winding; a first synchronous switch coupled to the transformer secondary, and a second synchronous switch coupled to the transformer secondary. The system further comprises a state retention device coupled to the second synchronous switch, the state retention device being operative to allow the second synchronous switch to remain in a conduction state when the voltage across the transformer secondary approaches zero volts; and a state release switch coupled to the second synchronous switch, the state release switch being operative to direct the second synchronous switch to switch to a non-conducting state when the first synchronous switch is directed to switch to a conducting state.